Pulling roll material for manufacture of sheet glass

ABSTRACT

A method of making sheet glass using a pulling roll comprised of a high-temperature millboard material. The millboard comprises aluminosilicate refractory fiber, silicate, mica, and kaolin clay. A method of manufacturing a pulling roll is disclosed, together with a roll produced by the methods disclosed herein. The method comprises forming a pulling roll and densifying at least a portion of the pulling roll by exposing to the pulling roll to high temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 11/655,478, filed on Jan. 19, 2007, now allowed asU.S. Pat. No. 7,507,194, which, in turn, is a divisional application ofU.S. patent application Ser. No. 12/368,794, filed on Feb. 10, 2009, nowallowed as U.S. Pat. No. 7,842,632, the contents of both of which arerelied upon and incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to the manufacture of sheet glass. Moreparticularly, the present invention relates to millboard materials andpulling rolls for use in the manufacture of sheet glass by, for example,the overflow downdraw fusion process.

BACKGROUND

Pulling rolls are used in the manufacture of sheet glass to applytension to the ribbon of glass from which the sheets are formed and thuscontrol the nominal sheet thickness. For example, in the overflowdowndraw fusion process (see Dockerty, U.S. Pat. Nos. 3,338,696 and3,682,609), pulling rolls are placed downstream of the tip or root ofthe fusion pipe and are used to adjust the rate at which the formedribbon of glass leaves the pipe and thus determine the nominal thicknessof the finished sheet.

A successful pulling roll needs to meet a number of conflictingcriteria. First, the roll needs to be able to withstand the hightemperatures associated with newly formed glass for substantial periodsof time. The longer a roll can last in such an environment the better,since roll replacement reduces the amount of finished glass a givenmachine can produce and thus increases the ultimate cost of the glass.

Second, the roll must be able to produce sufficient pulling force tocontrol glass thickness. In order not to damage the central portion ofthe ribbon that becomes the usable finished glass, the roll can onlycontact the ribbon over a limited area at its edges. Thus, the requiredpulling forces must be generated using only this area. However, theforces applied to the glass cannot be too large since this can createsurface damage which can propagate into the usable central portion ofthe ribbon. Accordingly, the roll must achieve a balance betweenapplying too little and too much force to the edge regions of the glass.

Third, the millboard material used in the construction of pulling rollsmust be hard enough to resist process damage due to broken glass duringproduction for extended periods of time.

Fourth, the pulling roll must not give off excessive amounts ofparticles, which can adhere to the glass and form surface defects knownas onclusions. For glass that is to be used in demanding applications,such as substrates for flat panel displays, onclusions must be kept tovery low levels since each onclusion will typically represent adefective region of the finished product (e.g., one or more defectivepixels). Because of the hot environment in which pulling rolls operate,providing materials that can apply sufficient pulling forces to a glassribbon and yet not give off particles when hot is a difficult challenge.

Pulling rolls are preferably designed to contact the glass ribbon at itsouter edges, specifically, in regions just inboard of the thickenedbeads that exist at the very edges of the ribbon. A preferredconstruction for such rolls employs discs of a heat resistant material,such as millboard, which are mounted on a driven shaft. Examples of thisconstruction can be found in Moore, U.S. Pat. No. 3,334,010, Asaumi etal., U.S. Pat. No. 4,533,581, and Hart et al., U.S. Pat. No. 5,989,170,which are incorporated by reference in their entirety and for thespecific purpose of describing examples of construction for pullingrolls.

Millboard materials have been used commercially for many years asthermal insulation in gaskets, linings for fire-safe cabinets, and inthe glass making industry as float roll covering materials. Earlymillboard compositions, such as those described in U.S. Pat. Nos.1,594,417, 1,678,345, and 3,334,010, often contained cement binders andasbestos fibers to strengthen the resulting millboard and provide heatresistance in high-temperature applications. Health concerns related tothe use of asbestos led to the development of asbestos-free millboardmaterials. U.S. Pat. No. 4,244,781, for example, discloses a millboardcomposition containing ceramic and organic fibers, pyrophyllite, and aninorganic binder. Similarly, U.S. Pat. No. 4,308,070 discloses amillboard containing a combination of cellulose fiber, barium sulphate,cement, and inorganic fiber.

Millboards comprised of washed ceramic fiber and incorporating variousfillers and functional components have also been used as roll coveringsfor float line rolls in the manufacture of glass. These washed ceramicmaterials frequently contain approximately twenty or more percent ofunfiberized material, or shot, of a size less than 100 mesh (0.0059inches). This unfiberized material can cause microscopic defects in theglass sheet as it passes over the float line rolls. Once the binder isremoved, these millboard materials can also become dusty and potentiallycreate onclusions on the glass sheets.

Existing pulling rolls have not been able to fully satisfy the competingcriteria of long high temperature life, controlled force application,hardness, and low contamination. Thus, there is a need in the art toobtain a pulling roll that achieves higher levels of such performancethan existing pulling rolls.

SUMMARY

The present invention relates to pulling rolls for glass manufacture,and more particularly to millboard materials used in the manufacture ofpulling rolls.

In a first aspect, the present invention provides a pulling roll forglass manufacture comprising at least one millboard piece, wherein theat least one millboard piece comprises: from about 5 to about 30 partsby weight aluminosilicate refractory fiber; from about 10 to about 30parts by weight silicate; from about 5 to about 25 parts by weight mica;and from about 10 to about 35 parts by weight kaolin clay; wherein thecombination of a, b, c, and d comprise at least 85 weight percent of themillboard piece.

In a second aspect, the present invention provides a method formanufacturing a pulling roll, comprising providing at least onemillboard piece in the form of a pulling roll, comprising from about 5to about 30 parts by weight aluminosilicate refractory fiber; from about10 to about 30 parts by weight silicate; from about 5 to about 25 partsby weight mica; and from about 10 to about 35 parts by weight kaolinclay; wherein the combination of a, b, c, and d comprise at least 85weight percent of the millboard; and densifying at least a portion ofthe millboard piece by exposing the millboard piece to a temperature offrom about 650° C. to about 1,000° C.

In a third aspect, the present invention provides a millboard comprisingfrom about 5 to about 30 parts by weight aluminosilicate refractoryfiber; from about 10 to about 30 parts by weight silicate; from about 5to about 25 parts by weight mica; and from about 10 to about 35 parts byweight kaolin clay; wherein the combination of a, b, c, and d compriseat least 85 weight percent of the millboard.

In still another aspect, the present invention provides a pulling rollproduced by the methods of the present invention.

In still another aspect, the present invention provides a pulling rollwherein at least a portion of the pulling roll comprises mullite.

In still another aspect, the present invention provides a pulling rollwherein at least a portion of the pulling roll comprises cristobalite.

Additional aspects of the invention will be set forth, in part, in thedetailed description and any claims that follow, and in part will bederived from the detailed description, or can be learned by practice ofthe invention. The advantages described below will be realized andattained by means of the elements and combinations particularly pointedout in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory only and are not restrictive of the inventionas disclosed.

DETAILED DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description, examples, and claims, and their previousand following description. However, before the present articles and/ormethods are disclosed and described, it is to be understood that thisinvention is not limited to the specific articles and/or methodsdisclosed unless otherwise specified, as such can, of course, vary. Itis also to be understood that the terminology used herein is for thepurpose of describing particular aspects only and is not intended to belimiting.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are products of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein.

The following description of the invention is provided as an enablingteaching of the invention in its currently known embodiment. To thisend, those skilled in the relevant art will recognize and appreciatethat many changes can be made to the various aspects of the inventiondescribed herein, while still obtaining the beneficial results of thepresent invention. It will also be apparent that some of the desiredbenefits of the present invention can be obtained by selecting some ofthe features of the present invention without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations to the present invention are possible andcan even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “millboard” includes aspects having two or moresuch millboards, unless the context clearly indicates otherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another aspect includes from the one particular value and/orto the other particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

References in the specification and concluding claims to parts byweight, of a particular component in a composition or article, denotethe weight relationship between the component and any other componentsin the composition or article for which a part by weight is expressed.Thus, in a compound containing 2 parts by weight of component X and 5parts by weight component Y, X and Y are present at a weight ratio of2:5, and are present in such ratio regardless of whether additionalcomponents are contained in the compound.

As used herein, a “wt. %” or “weight percent” or “percent by weight” ofa component, unless specifically stated to the contrary, is based on thetotal weight of the composition in which the component is included.

“Shot” refers to unfiberized material.

“Mullite” is a term known to those of skill in the art and refers to anatural or synthetic form of aluminum silicate that is stable attemperatures as high as 1600° C. and exhibits a low thermal expansioncoefficient and good mechanical strength.

“Cristobalite” is a term known to those of skill in the art and refersto a form of silica stable between 1,470° C. and its melting point of1,728° C. As used herein, cristobalite also includes a variation ofcristobalite known as high-cristobalite, which occurs above 268° C. butis only stable above 1,470° C. and which can crystallize and persistmetastably at lower temperatures.

As used herein, “compressibility” refers to the relative volume changeof a material as a response to an applied pressure. For example,compressibility of a pulling roll refers to the change in thickness ofthe assembled millboard pieces, or length of the assembled pulling roll,upon application of a compressive axial force.

As used herein, “recovery” refers to the ability of a compressedmaterial to expand after removal of an applied pressure. For example,recovery of a pulling roll refers to the expansion in thickness ofmillboard pieces upon either removal of an axial compressive force orupon elongation of the pulling roll shaft by, for example, thermalexpansion.

As briefly introduced above, the present invention provides an improvedpulling roll that, for example, can be useful in the manufacture ofsheet glass. Among other aspects described in detail below, theinvention comprises the use of millboard material containingaluminosilicate refractory fiber, silicate, mica, and kaolin clay, inthe manufacture of sheet glass.

Millboard

Millboard materials are often used as thermal insulation materials invarious industries, including glass manufacture. Millboard articles aretypically produced by creating a slurry of the desired components, usinga rotating screened cylinder to effect uptake and dewatering of thecomponents, transferring the dewatered components to a synthetic feltand then to an accumulator roll, where layers of the slurry areaccumulated upon one another to a desired thickness. These accumulatedlayers can be slit, removed, and formed into flat sheets of desireddimensions for subsequent use. After and during forming, the millboardsheet can be compressed by rollers to give it a uniform thickness. Theresulting millboard sheet can subsequently be heated to remove residualmoisture. U.S. Pat. Nos. 1,594,417, 1,678,345, 3,334,010, 4,487,631, and5,989,170, describe various compositions and methods for millboardmanufacture, and are incorporated by reference in their entirety and forthe specific purpose of describing methods of manufacture for millboardarticles. One of skill in the art could readily determine appropriateprocess conditions for the manufacture of a millboard article.

Aluminosilicate Refractory Fiber

In one aspect, the aluminosilicate refractory fiber is any refractoryfiber comprised substantially of an aluminosilicate material. Naturallyoccurring or synthetic refractory fiber can be used. Specifically,refractory fiber derived from kaolinite or kaolin based materials can beused. In another aspect, the naturally occurring refractory fiberderived from a kaolin based material can contain impurities such as ironoxide, titanium dioxide, and sodium oxide. In one aspect, the refractoryfiber of the present invention can have a length of, for example, up to5 microns, a diameter of, for example, up to 3 microns, and an aspectratio of, for example, 5 to 1. It is preferable that the refractoryfiber is substantially free of shot, or unfiberized material. It ispreferable that the refractory fiber not melt at temperatures up toabout 1,760° C., and retain physical and chemical integrity whensubjected to continuous temperatures of up to about 1,260° C. Therefractory fiber can be a FIBERFRAX® material, for example, FIBERFRAX®6000, available from Unifrax Corporation, Niagara Fall, N.Y., USA, whichis derived from kaolin and is comprised of from about 45% to about 51%alumina, from about 46% to about 52% silica, less than about 1.5% ironoxide, less than about 2% titanium dioxide, less than about 0.5% sodiumoxide, has an average fiber diameter of about 1.5 to about 2.5 microns,and contains from about 45% to about 55% fiberized material. One ofskill in the art could readily choose an appropriate aluminosilicaterefractory fiber.

The aluminosilicate refractory fiber can be from about 5 to about 30parts by weight, preferably from about 10 to about 30 parts by weight,and more preferably from about 20 to about 30 parts by weight of thecombination of aluminosilicate refractory fiber, silicate, mica, andkaolin clay, for example, about 5, 6, 8, 10, 15, 20, 25, 26, 28, 29, or30 parts by weight of the above combination. Expressed in weight percentof the total composition, the refractory fiber can be from about 5.5 toabout 33.3 weight percent, preferably from about 11.3 to about 33.3weight percent, and more preferably from about 22.6 to about 33.3 weightpercent, for example, 5.5, 7, 10, 15, 20, 25, 27, 30, or 33.3 weightpercent of the total millboard composition.

Silicate

The silicate can be a magnesium silicate, a rock wool, or a combinationthereof. Naturally occurring or synthetic silicate material can be used.The silicate can be a forsterite mineral or a synthetic forsteriteobtained by calcination of chrysotile asbestos fibers. It is preferablethat the silicate be a magnesium silicate, such as a FRITMAG™ magnesiumsilicate, available from 4372077 Canada Inc., Sherbrooke, Qc, Canada.Alternatively, the silicate can be a sepiolite magnesium silicate. Ifthe silicate is a sepiolite magnesium silicate, precautions should betaken as this material can contain asbestos fibers. One of skill in theart could readily choose an appropriate silicate material.

The silicate can be from about 10 to about 30 parts by weight,preferably from about 15 to about 25 parts by weight, and morepreferably from about 15 to about 20 parts by weight of the combinationof aluminosilicate refractory fiber, silicate, mica, and kaolin clay,for example, about 10, 11, 12, 15, 16, 17, 20, 25, or 30 parts by weightof the above combination. Expressed in weight percent, the silicate canbe from about 11.1 to about 33.3 weight percent, preferably from about16.9 to about 28.2 weight percent, and more preferably from about 16.9to about 22.6 weight percent, for example, 11.1, 15, 20, 25, 27, 30, or33.3 weight percent of the total millboard composition.

Mica

The mica can be any phyllosilicate of the mica group that is a sheetsilicate in the form of parallel sheets of silicate tetrahedral, witheither Si₂O₅ or a 2 to 5 ratio, for example, biotite, muscovite,lepidolite, phlogopite, or illite. In one aspect, the mica is a highsurface area mica that is substantially free if impurities and exhibitsthermal stability, low ignition loss, and is inert. The mica ispreferably a phlogopite flake mica, such as SUZORITE® 325-S, availablefrom Suzorite Mica Products, Inc. (Suzor Township, Quebec, Canada). Oneof skill in the art could readily choose an appropriate mica material.

The mica can be from about 5 to about 25 parts by weight, preferablyfrom about 10 to about 25 parts by weight, and more preferably fromabout 15 to about 25 parts by weight of the combination ofaluminosilicate refractory fiber, silicate, mica, and kaolin clay, forexample, about 5, 6, 8, 10, 15, 20, 21, 22, 24, or 25 parts by weight ofthe above combination. Expressed in weight percent, the mica can be fromabout 5.5 to about 27.8 weight percent, preferably from about 11.3 toabout 27.8 weight percent, and more preferably from about 16.9 to about27.8 weight percent, for example, 5.5, 7, 9, 15, 19, 25, 27, or 27.8weight percent of the total millboard composition.

Kaolin Clay

The kaolin clay can be any kaolin or china clay material, such askaolinite. The kaolin clay is preferably intermediate grainedair-floated Kaolin clay, such as Allen clay, available fromKentucky-Tennessee Clay Co., Sandersville, Ga., USA. One of skill in theart could readily choose an appropriate kaolin clay.

The kaolin clay can be from about 10 to about 35 parts by weight,preferably from about 20 to about 35 parts by weight, and morepreferably from about 25 to about 35 parts by weight of the combinationof aluminosilicate refractory fiber, silicate, mica, and kaolin clay,for example, about 10, 11, 13, 20, 25, 30, 31, 32, or 35 parts by weightof the above combination. Expressed in weight percent, the kaolin claycan be from about 11.1 to about 39.5 weight percent, preferably fromabout 22.6 to about 39.5 weight percent, and more preferably from about28.2 to about 39.5 weight percent, for example, 11.1, 13, 15, 20, 30,33, 38, or 39 weight percent of the total millboard composition.

Other Materials

The millboard material can further comprise a functional component. Inone aspect, the functional component comprises a cellulose material, astarch material, a colloidal silica, or a mixture thereof. Functionalcomponents can be useful in the formation of millboard articles. Afunctional component can combust or decompose during heating or use of amillboard article at typical pulling roll operating temperatures. In oneaspect, a functional component can be a processing aid, such as aprocessed wood pulp cellulose fiber. A functional component can also bea binder, such as a cationic potato starch, for example, Empresol N,available from American Key Products, Inc, Kearney, N.J., USA, or acolloidal silica, such as an alkaline colloidal silica solution, forexample, LUDOX®-Nalco 1140, available from Nalco Chemical Co.,Naperville, Ill., USA.

A functional component can be up to about 15 weight percent of themillboard material.

It is preferable that the millboard material is substantially free ofasbestos, unfiberized material, and small crystalline silica particles.The millboard material preferably contains less than about 0.5 weightpercent, more preferably less than about 0.1 weight percent, and mostpreferably is free of crystalline silica. The millboard material alsopreferably contains less than about 0.8 weight percent, more preferablyless than about 0.3 weight percent, and most preferably is free oftitanium dioxide.

Overall Millboard Composition

The millboard of the present invention is comprised of from about 5 toabout 30 parts by weight aluminosilicate refractory fiber; from about 10to about 30 parts by weight silicate; from about 5 to about 25 parts byweight mica; and from about 10 to about 35 parts by weight kaolin clay;wherein the combination of the aluminosilicate refractory, silicate,mica, and kaolin clay comprise at least 85 weight percent of themillboard piece, preferably at least 95 weight percent of the millboardpiece. The overall millboard composition can further comprise afunctional component as described above. The functional component cancombust or decompose during heating to temperatures typical for pullingroll operation and glass manufacture, affecting the percentage ofindividual components in the overall millboard composition. Weight lossdue to combustion or decomposition of functional component can be fromabout 0 to about 15 weight percent. In one aspect, the millboardcomposition loses from about 8 to about 15 weight percent upon heating.In another aspect, the millboard composition loses about 10 weightpercent during heating.

In one aspect, a preferred millboard composition, after heating,comprises from about 20 to about 30 weight percent, preferably about 26weight percent aluminosilicate refractory fiber; from about 10 to about20 weight percent, preferably about 15 weight percent silicate; fromabout 14 to about 25 weight percent, preferably about 20 weight percentmica; from about 28 to about 35 weight percent, preferably about 31weight percent kaolin clay, and from about 5 to about 10 weight percent,preferably about 8 weight percent LUDOX®.

In one aspect, a preferred millboard composition has a temperatureresistance of greater than about 1,000° C.

The compressibility of a pulling roll is dependent upon the density ofthe millboard pieces from which the pulling roll is formed. It isdesirable that a pulling roll, and thus the millboard material, exhibitlow compressibility, for example, between about 15 and about 30 percentat 25° C., and less than 5 percent at about 110° C. It is also desirablethat a millboard material exhibit high recovery, for example, greaterthan about 30 percent, preferably greater than about 40 percent.Millboard materials possessing such recovery percentages can expand uponremoval of the axial compressive force placed on a pulling roll or uponelongation of the pulling roll shaft as a result of thermal expansion,thus preventing separation of the millboard pieces that form the pullingroll.

In contrast, a commercially available millboard material, NichiasSD-115, available from Nichias Corporation, Tokyo, Japan, is believed tobe comprised of 10-20 percent refractory ceramic fiber, 40-50 percentmica, and 40-50 percent clay. The Nichias SD-115 material has atemperature resistance of only about 800° C., a weight loss upon heatingof 14-16%, compressibility at 25° C. of 10-17%, and a recovery at 760°C. of 35-40%.

As described here and in the examples below, the inventive millboardexhibits a higher temperature resistance, a lower weight loss uponheating, and/or a higher recovery at 760° C.

Pulling Roll

A pulling roll, for use in the manufacture of sheet glass, can beproduced from a millboard, as described above. The millboard can be cutinto pieces and the pieces mounted on a shaft in face-to-face contact.The outer surface of each piece forms a portion of the exterior surfaceof the pulling roll. At least a portion of the exterior surface of thepulling roll can be adapted to contact the glass sheet. The portion ofthe pulling roll adapted to contact the glass sheet typically has aShore D hardness at room temperature of between 30 and 55, preferablybetween 40 and 55.

It should be appreciated that a variety of pulling roll configurationsexist in the literature and are suitable for use in the manufacture ofsheet glass. U.S. Pat. No. 6,896,646 describes pulling rolls for glasssheet manufacture, and is incorporated by reference in entirety and forthe specific purpose of describing methods of producing a pulling rollfrom millboard materials. The present invention is not limited to aparticular pulling roll configuration or arrangement, and one of skillin the art could readily choose an appropriate pulling rollconfiguration.

In a typical configuration, a pair of pulling rolls engage a glass sheetformed by an overflow downdraw process, wherein at least a portion ofthe outer surface of the pulling rolls contacts the glass sheet. Apulling roll can also include a shaft, which can carry a plurality ofmillboard pieces held in place by collars that can apply an axialcompressive force to the millboard pieces when affixed to the shaft. Anassembled pulling roll can include a bearing surface positioned on atleast one end of the shaft. A pulling roll can also include a portionspecifically adapted for contacting a glass sheet, wherein the exteriorsurface of the pulling roll extends a further distance from the shaftthan does the surrounding portion of the pulling roll. Such aconfiguration can reduce the possibility of particles from the pullingroll becoming deposited on the glass sheet as conclusions.

The millboard pieces can be pre-fired prior to assembly to form thepulling roll so that they exhibit substantially no compositional ordimensional changes when exposed to the temperatures at which the rollsoperate. For example, millboard pieces can be heated in a pre-firingstep to a temperature of from about 650° C. to about 1,000° C.,preferably from about 760° C. to about 1,000° C., and held for a periodof at least two hours. The millboard pieces can then be cooled toambient temperature and assembled to form a pulling roll. Functionalcomponents present in the millboard material, such as cellulose, can becombusted by heating in such a pre-firing step. Alternatively, thepulling roll can be used without a pre-firing step. If the millboardmaterial from which the pulling roll is formed comprises combustiblefunctional components, the compressive forces used to assemble thepulling roll can require adjustment to compensate for the combustedfunctional component. Other pre-firing times and temperatures can, ofcourse, be used in the practice of the invention so long as they providea finished pulling roll whose composition is stable at the rolls'operating temperature.

Densification and Formation of Mullite and/or Cristobalite

One aspect of the inventive pulling roll is that it is sufficiently hardto resist process damage, such as broken glass due to checks duringproduction for extended periods of time. Sideways movement of the glassduring production is often related to separation of the millboard piecesthat comprise the pulling roll. Checks, or embedded glass particles inthe surface of a pulling roll can occur when softer millboard materialsare employed. Upon exposure to operating temperatures, for example, fromabout 650° C. to about 1,200° C., a portion of the pulling rolldensifies, wherein the density of that portion of the roll is greaterthan that of the pulling roll as originally formed. Initially,densification can occur at the outer surface of the pulling roll incontact with the glass, or in various geometries, as determined by thepulling roll configuration and the specific glass manufacturingconditions and temperatures. The rate of densification over time isbased on the temperatures to which the pulling roll is exposed.Densification can be measured via Shore D hardness values at the pullingroll surface using commercially available equipment such as a durometer.It is preferable that the portion of the pulling roll that will contactthe glass sheet be harder than traditional millboard and pulling rollmaterials, and thus more resistant to process damage and embedded glass.

Upon further exposure to temperatures of about 1,000° C. and greater, aportion of the pulling roll can form mullite, cristobalite, or acombination thereof. The portion of the pulling roll that can formmullite and/or cristobalite can vary depending on the configuration ofthe pulling roll and the temperatures to which the roll is exposed, butwill typically be the exterior portion of the pulling roll. It ispreferable that the portion of the pulling roll that will contact theglass sheet also form a mullite layer, a cristobalite layer, or acombination layer comprising mullite and cristobalite.

Densification and formation of mullite is beneficial to the performanceof a pulling roll. Pulling rolls that are sufficiently hard to resistprocess damage have been found to achieve longer service lives thantraditional pulling rolls, without requiring the application ofexcessive force to the glass sheet and without generating high levels ofparticulate contamination. The inventive pulling roll can achieve aservice life of from 40 to in excess of 100 days, preferably in excessof 75 days, and most preferably in excess of 100 days.

The pulling roll of the present invention can satisfy one or more of thedemanding requirements described above. It is not necessary that thepulling roll of the present invention simultaneously satisfy all of therecited requirements. In one aspect, densification and/or formation ofmullite can allow the pulling roll to withstand the high temperaturesassociated with glass formation and provide a longer service life. Inanother aspect, densification and/or formation of cristobalite can allowthe pulling roll to withstand the high temperatures associated withglass formation and provide a longer service life. In another aspect,the surface of the pulling roll of the present invention can applypulling forces sufficient to control glass sheet thickness. In yetanother aspect, the composition of the pulling roll is sufficiently hardto resist process damage due to broken glass and does not give offexcessive particles that can create onclusions on glass sheetsmanufactured by a downdraw process.

EXAMPLES

To further illustrate the principles of the present invention, thefollowing examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how themillboard pulling rolls and methods claimed herein are made andevaluated. They are intended to be purely exemplary of the invention andare not intended to limit the scope of what the inventors regard astheir invention. Efforts have been made to ensure accuracy with respectto numbers (e.g., amounts, temperatures, etc.); however, some errors anddeviations can occur. Unless indicated otherwise, parts are parts byweight, temperature is ° C. or is at ambient temperature, and pressureis at or near atmospheric.

The exemplified pulling roll articles were evaluated for relevantphysical and performance properties, such as for example, hardness,compressibility, and recovery.

Example 1 Inventive Millboard A

In a first example, a millboard material was produced from thecomponents set forth in Table 1 below, using traditional fabricationtechniques.

TABLE 1 Inventive Millboard A Wt. Percent Component 1.5 Cellulose fiber24.5 FIBERFRAX ® 6000 Refractory Fiber 15.0 Fritmag magnesium silicate19.0 SUZORITE ® 325-S mica 30.0 Kaolin (Allen) Clay 2.5 Empresol Nstarch 7.5 LUDOX ® colloidal silica

A piece of the inventive millboard produced above was subsequentlyanalyzed at two temperatures for density, thickness, hardness, andcompression. The results of this analysis are summarized in Table 2below. Hardness values were determined using according to ASTM D2240with a Shore durometer, available from Wilson Instruments, Norwood,Mass., USA. Compressibility and recovery values were determinedaccording to ASTM F36.

TABLE 2 Physical Properties of Inventive Millboard A Temperature Units110° C. 760° C. Density g/cm³ 1.006 0.974 Thickness mm 6.01 5.85Hardness Shore D 48 35 Compressibility % 4.57 12.67 Recovery % 45.2240.31

An examination of the data set forth in Table 2 indicates, inparticular, that the millboard composition exhibits a Shore D hardnessvalue sufficiently high to provide advantages in handling and processingglass sheet without incurring process damage due to broken glass. Inaddition, the low compressibility and high recovery rate of theinventive millboard material suggest that it is well suited for use in apulling roll. The high recovery rate indicates that the compressedmillboard material can act as a spring against the collar of a pullingroll during fabrication and at operating temperatures.

Example 2 Comparative Millboard

In a second example, Inventive Millboard A was compared to a NichiasSD-115 material. Table 3 details the typical range of physicalproperties for both the Inventive Millboard A and the Nichias SD-115material.

TABLE 3 Comparison of Inventive Millboard A and Nichias SD-115 Inv.Millboard Property A Nichias SD-115 Temperature Resistance ≧1000° C.800° C. Weight Loss upon firing at 760° C. 10.9-14.4% 14.0-16.0%Incremental Weight Loss, 650° C. to 0.3 % 1.8% 1,000° C. Shore DHardness at 25° C.   34-49   35-50 Compressibility at 110° C.  4.2-15.1%  10-12% Recovery at 760° C. 30.1-43.4%   35-40%

As detailed in Table 3 above, Inventive Millboard A exhibits a highertemperature resistance than the comparative Nichias SD-115 material. Theinventive millboard also exhibits a lower weight loss upon firing ofpunched millboard discs at 760° C. The incremental weight loss between650° C. and 1,000° C., as determined by thermogravimetric analysis, isindicative of the amount of material lost to combustion or decompositionduring operation of a pulling roll. Materials having higher incrementalweight losses will typically require adjustment of the compression of apulling roll to prevent disc separation. Alternatively, materialsexhibiting high recovery can expand to fill the volume lost tocombustion, decomposition, or upon elongation of the pulling roll shaftby, for example, thermal expansion. The inventive millboardadvantageously exhibits a substantially lower incremental weight loss,together with a higher recovery value. The inventive millboard also hasa lower compressibility than the SD-115 material, indicating that it ismore suitable for use in producing a pulling roll.

Example 3 Inventive Pulling Roll

In a third example, pulling rolls manufactured from inventive MillboardA and from a commercially available millboard material, Nichias SD-115,were used in the manufacture of sheet glass. Glass sheet production runswere conducted using the inventive pulling roll and the comparativeNichias pulling roll for 24 hours. The results of the experimentalproduction runs are detailed in Table 4 below:

TABLE 4 Glass Sheet Production Runs Inventive Millboard Nichias SD- A115 Select Sheets, % 97.7 94.5 Checks/Cracks, % 0 1.1 Sheet ThicknessSimilar Similar Length of Run 24 hours 24 hours Stress, Range Max 109.42135.19 Absolute Max 97.60 −107.33 200 mm Range 95.75 104.74 Max

As indicated in Table 4, the pulling roll fabricated from InventiveMillboard A performed well in the sheet glass production run. The termselect sheets, as used in Table 4, refers to sheet glass pieces passingquality control standards and that are suitable for distribution. Theproduction run using the Inventive Millboard A pulling roll generated asignificantly higher percentage of select sheets than the comparable runusing the Nichias pulling roll. Additionally, the Inventive Millboard Apulling roll did not generate any glass sheet containing checks orcracks, while the Nichias pulling roll generated 1.1% glass sheetscontaining checks or cracks. Thickness of the sheet glass produced fromboth pulling rolls was comparable.

The increased production of select sheets and lower level of productiondefects, i.e. checks/cracks, represent significant commercial advantagesfor the pulling roll fabricated from the inventive millboard material ofthe present invention. Table 4 also illustrates the reduced stressapplied to the glass sheet from the inventive pulling roll. Stress ismeasured along all four edges of the sheet, so some common metrics foranalyzing this data are shown in Table 4. Since stress can be eitherpositive or negative, it is typical for the data to be evaluated basedon absolute values, ranges of values, and maximum of values (worst casescenario). Low stress is considered to be good and customers typicallydictate the range of stress they can tolerate.

Throughout this application, various publications are referenced. Thedisclosures of these publications in their entireties are herebyincorporated by reference into this application in order to more fullydescribe the compounds, compositions and methods described herein.

Various modifications and variations can be made to the compounds,compositions and methods described herein. Other aspects of thecompounds, compositions and methods described herein will be apparentfrom consideration of the specification and practice of the compounds,compositions and methods disclosed herein. It is intended that thespecification and examples be considered as exemplary.

1. A method of making a sheet glass using a pulling roll comprising atleast one millboard piece, wherein the at least one millboard piececomprises: a. from about 5 to about 30 parts by weight aluminosilicaterefractory fiber, said aluminosilicate refractory fiber having a lengthup to 5 microns; b. from about 10 to about 30 parts by weight silicate;c. from about 5 to about 25 parts by weight mica; and d. from about 10to about 35 parts by weight kaolin clay; wherein the combination of a,b, c, and d comprise at least 85 weight percent of the millboard piece.2. A method of claim 1, wherein at least a portion of the pulling rollcomprises mullite.
 3. A method of claim 1, wherein at least a portion ofthe pulling roll comprises cristobalite.
 4. A method of claim 1, whereinat least a portion of the outer surface of the pulling roll comprisesmullite, and wherein the mullite portion of the outer surface ispositioned to contact a glass sheet.
 5. A method of claim 1, wherein atleast a portion of the outer surface of the pulling roll comprisescristobalite, and wherein the cristobalite portion of the outer surfaceis positioned to contact a glass sheet.
 6. A method of claim 1, which isa fusion down-draw process.